On-line gas chromatograph with sample preparation, concentration, and calibration apparatus for measuring trace organic species from combustor flue gas

Abstract

Apparatus and method for periodic analysis of trace amounts of volatile organic compounds in a waste gas provide for feeding of a sample of a waste gas first to an organics concentrator which isolates the volatile organic compounds from the waste gas sample and prepares a concentrated sample for feed to a gas chromatograph. In normal operation, a waste gas is continuously sampled from the waste gas source to produce a continuous waste gas flow through a switching valve and out a vent. A portion of that waste gas flow is periodically diverted by the switching valve and routed to the organics concentrator. The switching valve also receives calibration samples containing known concentrations of the volatile organic compounds prepared in a gas blender. It periodically feeds the calibration samples to the organics concentrator and the gas chromatograph for calibration of the system.

We claim: 
     
       1. A process for analyzing a waste gas stream to determine concentrations of volatile organic compounds contained in the waste gas stream, said process comprising: sampling the waste gas stream to establish a continuous waste gas flow through, in sequence, a waste gas line, a switching valve and a first vent line;   periodically diverting a portion of the waste gas flow at the switching valve;   feeding the diverted portion of the waste gas flow in a first direction through a sorbent in an organics concentrator, preferentially sorbing the volatile organic compounds on said sorbent, and venting, through a second vent line, the remaining waste gas essentially free of the volatile organic compounds;   periodically discontinuing the feed of the diverted waste gas flow through the sorbent and feeding a carrier gas through the sorbent in a second direction, opposite said first direction, for desorbing the volatile organic compounds from the sorbent thus forming a concentrated gas sample;   periodically feeding the concentrated gas sample to a gas chromatograph for analysis of the concentrated gas sample to determine concentrations of the volatile organic compounds;   blending various amounts of at least one of the volatile organic compounds with a gaseous carrier to form calibration samples containing known, different concentrations of the at least one volatile organic compound;   feeding the calibration samples through the sorbent in the organics concentrator for sorbing and desorbing of the calibration samples;   feeding the calibration samples desorbed from the sorbent to the gas chromatograph for analysis and calibration of the apparatus.   
     
     
       2. The process of claim 1 wherein said sorbing is conducted at ambient temperature and said desorbing is with heating of the sorbent. 
     
     
       3. The process of claim 1 further comprising regulating the flow of diverted waste gas through the sorbent and the feed of calibration samples through the sorbent to the same constant gas flow rate. 
     
     
       4. The process of claim 1 wherein a gaseous blend of a plurality of the volatile compounds in known concentrations is blended with the gaseous carrier to form the calibration samples, each containing different concentrations of the plural volatile organic compounds. 
     
     
       5. The process of claim 1 wherein the calibration samples are fed to the organics concentrator through the switching valve. 
     
     
       6. The process of claim 1 wherein the calibration samples are introduced into a control valve located in the waste gas line at a point adjacent the waste gas stream, the control valve switching between a sampling position where the waste gas is admitted to the waste gas line with the calibration samples blocked therefrom and a calibration position where the calibration samples are admitted to the waste gas line with the exhaust gas blocked therefrom. 
     
     
       7. The process of claim 1 further comprising scrubbing the continuous waste gas flow to remove acidic gases. 
     
     
       8. An apparatus for periodic analysis of trace amounts of volatile organic compounds in a waste gas, comprising: a probe for extracting a sample from a stream of the waste gas;   an organics concentrator containing a sorbent having preferential affinity for the volatile organic compounds;   a waste gas line connecting said probe to a first vent line;   a switching valve between said waste gas line and said first vent line for diverting, in a sampling mode, a portion of the waste gas flow to said vent to said organics concentrator, the waste gas portion flowing through said sorbent in a first direction, in said sampling mode, and exiting said organics concentrator through a second gas vent line;   a source of a carrier gas for feeding the carrier gas through the sorbent in a second direction, opposite the first direction, to purge the volatile organic compounds from the sorbent thereby forming a gas sample stream;   a gas chromatograph for receiving the gas sample stream and for analyzing the gas sample stream to determine concentrations of volatile organic compounds contained therein;   a gas blender for blending at least one of the volatile organic compounds with a carrier gas to form a plurality of calibration gas streams containing different concentrations of the at least one volatile organic compounds; and   a calibration gas line for feeding the calibration gas streams to said organics concentrator in a calibration mode.   
     
     
       9. The apparatus of claim 8 wherein said calibration gas line connects said gas blender to said switching valve. 
     
     
       10. The apparatus of claim 8 wherein said organics concentrator contains a heater for heating said sorbent during the purging of volatile organic compounds from the sorbent. 
     
     
       11. The apparatus of claim 8 further comprising a three-way valve in said waste gas line adjacent said probe and a system calibration line for feeding the calibration gas streams from said gas blender to said three-way valve. 
     
     
       12. The apparatus of claim 8 further comprising a sample pump in said waste gas line and a vacuum pump in said second vent line. 
     
     
       13. The apparatus of claim 8 further comprising a mass flow controller in said second vent line for controlling flow of the exhaust gas through said organics concentrator to a constant gas flow rate. 
     
     
       14. The apparatus of claim 8 further comprising, in said waste gas line, a heated filter for removing particulates from the waste gas and a gas scrubber for removing acid gases from the waste gas. 
     
     
       15. The apparatus of claim 8 wherein said gas chromatograph has a flame ionization detector, an electron capture detector and a chromatograph column which receives the gas sample stream, and wherein said gas chromatograph splits the gas sample stream exiting the chromatograph column to simultaneously feed portions of the gas sample stream to both the flame ionization detector and the electron capture detector. 
     
     
       16. The apparatus of claim 8 wherein said gas chromatograph has a mass selective detector.